A step of mass-producing plant bodies (seedlings) having a trait appropriate for a purpose and being homogeneous in quality is unavoidably required in industrially using the plant bodies for the purpose of agricultural production, afforestation, breeding, etc. A conventional cutting propagation and a tissue culture method created by recent development in biotechnology are useful as means for mass-producing plant bodies. Employing those methods makes it possible not only to merely mass-produce seedlings, but also to rapidly produce a large amount of plant bodies having the same trait, i.e., clone seedlings. In particular, in order to mass-produce an excellent line of a genetically modified tree, it is necessary to vegetatively propagate seedlings of the selected line with efficiency.
In a case of the cutting propagation, a plant body is produced by preparing a cutting of a branch or a stalk, or alternatively, a terminal bud, an axillary bud, a leaf, a seed leaf, a hypocotyl, or the like in some cases, from an individual plant to be propagated, and then planting and rooting the cutting to a cutting bed or a medium. Meanwhile, in a case of mass-producing plants by use of the tissue culture method, an appropriate tissue cut from an individual plant to be propagated is cultured. Then, an adventitious bud, an adventitious embryo, a shoot primordium, or a foliage (shoot) grown from a tissue of any of them is collected and is rooted. That is, rooting is necessary no matter which method is selected to produce clone seedlings.
Accordingly, rooting ability of a plant tissue affects productivity of a clone seedling. In particular, if a plant species is low in rooting ability, the low rooting ability becomes a serious hindrance against industrial application of clone seedlings of the plant species.
It is known that various substances influence rooting of a plant tissue. One example of such a substance whose influence on rooting has been studied is ethylene that a plant tissue itself emits. In order to reduce generation of ethylene, it has been proposed to culture a tissue of an adventitious bud or the like with a medium in which silver nitrate is added (see Patent Literature 1). This method can reduce a mortality rate of the plant tissue to some extent, however, rooting of the plant tissue is suppressed or delayed. Therefore, this method would be effective to merely slightly improve the plant tissue in rooting rate, or this method would even inhibit the rooting rate of the plant tissue.
Furthermore, Patent Literature 2 discloses that (I) a rooting rate of eucalyptus is particularly improved by using a medium containing (i) silver thiosulfate (AgS4O6, Silver Thiosulfate Complex; hereinafter, abbreviated simply as “STS”) serving as a source of silver ions and (ii) an ascorbic acid serving as an antioxidant, and (II) red light is more preferably used as a light source than white light. However, rooting rates of some plant species and some particular eucalyptus lines with useful trait cannot be improved sufficiently by these improvements, thereby resulting in low rooting rates that lead to low productive efficiency of seedlings. This has been a big problem for this technique.
Meanwhile, there is a report that glutathione influences differentiation of an adventitious embryo from a callus. For example, Patent Literature 3 discloses that culturing of calluses of a rice plant and a Russell prairie gentian in a medium containing glutathione promotes redifferentiation.
However, Patent Literature 4 discloses that addition of glutathione of 1 mM rather inhibits a callus of a tobacco plant from forming an adventitious bud, and addition of buthionine sulfoximine (BSO) which serves as a glutathione synthesis inhibitor remarkably increases the number of differentiation of adventitious buds per callus. This indicates that the influence of glutathione on redifferentiation of callus of a plant body differs depending on the kind of the plant body.
As described above, there has been no technique for obtaining practically enough quantity of clone seedlings from shoots; and for glutathione, it has been merely reported that glutathione influences formation of adventitious embryos of calluses of limited kinds of plant species. Under such circumstances, there has been desired an industrially useful technique for obtaining clone seedlings by efficiently rooting shoots.